It is known that various monoanhydrides of organic carboxylic acids, for example, maleic acid, phthalic acid, methyl-3,6-endomethylene tetrahydrophthalic acid and hexahydrophthalic acid, are useful as a curing agent for producing cured epoxy resins having a high thermal resistance. However, it is also known that when the epoxy resins are cured by using the above-mentioned conventional organic carboxylic monoanhydrides, the thermal resistance of the resultant cured epoxy resins is not always satisfactorily high for practical use at a high temperature. Furthermore, numerous articles in published literature relate to the fact that various dianhydrides of organic carboxylic acids, for example, pyromellitic acid and 3,3',4,4'-benzophenonetetracarboxylic acid, are useful as a curing agent for producing the cured epoxy resin having a high thermal resistance. However, the above-mentioned conventional organic carboxylic dianhydrides can be dissolved in the epoxy resins only at a temperature higher than the curing temperature of the epoxy resins or have a poor compatibility with the epoxy resins. That is, when the curing agent consists of the conventional carboxylic acid dianhydride alone, the curing agent can be completely dissolved only at a temperature of 190.degree. C. or more. Even when the curing agent can be completely dissolved in the epoxy resin, the resultant solution is so unstable that the entire amount or a portion of the solution is rapidly gelled at the dissolving temperature. Sometimes, when the solution is cooled and reaches a temperature of 180.degree. C. or less, the curing agent consisting of the conventional organic carboxylic dianhydride undesirably deposits from the solution. When the above-mentioned deposition of the curing agent occurs, the curing of the mixture of the epoxy resin with the curing agent results in formation of cracks in the resin during the curing operation or the resultant cured epoxy resin is brittle and has a poor tenacity. In order to prevent the deposition of the curing agent consisting of the conventional organic carboxylic dianhydride from the epoxy resin solution, it is necessary to mix a considerable amount of the conventional organic carboxylic monoanhydride and/or a non-reactive diluent. The monoanhydride and the non-reactive diluent are effective for lowering the temperature at which the curing agent can be completely dissolved in the epoxy resin and the temperature at which the curing agent can be completely dissolved in the epoxy resin and the temperature at which the curing agent can be deposited from the epoxy resin solution. However, the resultant cured epoxy resin, which contains a considerable amount of the conventional organic carboxylic monoanhydride and/or the non-reactive diluent, has a poor thermal resistance at a high temperature. Under the above-mentioned circumstances, a new curing agent which is highly compatible with the epoxy resins and capable of producing cured epoxy resins having an excellent thermal resistance even at a high temperature is strongly desired.